Use fields in hidden native DOM classes instead of getters/setters.

client/dom/generated/src/frog/File.dart

Index: client/dom/generated/src/frog/File.dart
diff --git a/client/dom/generated/src/frog/File.dart b/client/dom/generated/src/frog/File.dart
index 4c2fbc632ec94f0c66d726d6685c3b1f2a9ec883..cc1245fa37c81232734c66718d00b6a82395c0c5 100644
--- a/client/dom/generated/src/frog/File.dart
+++ b/client/dom/generated/src/frog/File.dart
@@ -1,13 +1,13 @@
 
 class _FileJs extends _BlobJs implements File native "*File" {
 
-  String get fileName() native "return this.fileName;";
+  final String fileName;
 
-  int get fileSize() native "return this.fileSize;";
+  final int fileSize;
 
-  Date get lastModifiedDate() native "return this.lastModifiedDate;";
+  final Date lastModifiedDate;
 
-  String get name() native "return this.name;";
+  final String name;
 
-  String get webkitRelativePath() native "return this.webkitRelativePath;";
+  final String webkitRelativePath;
 }